The present invention relates to a film carrier for simultaneously connecting external leads to the electrodes of electronic parts such as semiconductor elements, and also to a bonding method using this film carrier.
Recent times have seen the manufacture of semiconductor integrated circuits of increasing integration density, as a result of the progress made in improving the manufacturing techniques. This has made it possible to manufacture integrated circuits each of which now has more than 100 terminals. Progress in manufacturing has, not surprisingly, been accompanied by the demand for the development of a highly efficient technique for bonding the terminals of these highly integrated circuits to a circuit board. The so-called film carrier, which can help to bond the numerous terminals of a semiconductor integrated circuit element, at a time, to a circuit board, is now attracting much attention.
The film carrier comprises a base film made of resin, on which copper conductors are formed. While being pressed onto the terminals of the semiconductor element, the copper conductors are fused to the terminals of a semiconductor element. More specifically, bumps made of a lamination of titanium, nickel, palladium, and gold are formed on the aluminum terminals of the semiconductor element, and tin layers are deposited on the copper conductors formed on the base film. Both the bumps and the tin layers are heated to a high temperature ranging from 450.degree. to 500.degree. C., and the conductors are pressed onto the aluminum terminals, with a high pressure ranging from 200 to 1000 kg/cm.sup.2 (20 to 100 g for one 100 .mu.m.times.100 .mu.m terminal), thereby forming a gold-tin eutectic alloy. This eutectic alloy bonds the conductors to the terminals of the semiconductor element.
Among the various known bonding methods using film carriers is the tape automated bonding method, which is disclosed in Japanese Patent Disclosure Sho 57-152147. Using this method, gold bumps are transferred onto tin layers on the copper of a film carrier, and these leads are fused to the aluminum terminals of a semiconductor element, while being pressed onto the terminals.
The conventional bonding methods using a film carrier, however, have drawbacks.
Since the leads are heated to a high temperature and pressed onto the terminals of the semiconductor element with a high pressure, there is the possibility that the semiconductor element will be damaged. Further, when the conventional method is used when manufacturing color liquid-crystal displays (e.g., a-SiTFT displays), the organic color filters used in the displays will be heated to a temperature over their maximum operating temperature of 150.degree. C., while the leads of the film carrier and the terminals of the semiconductor elements used in the displays are being heated to the above-mentioned high temperature and high pressure. Consequently, this will result in the degradation of the color filters.
When the bonding method using a film carrier is used to bond the outer leads, such as the drive-signal terminals of a liquid-crystal device, these terminals and the leads of the film carrier are plated with a solder alloy, and the plated solder alloy is melted. The plated layers of the solder alloy may exfoliate from the terminals and the leads, or may develop cracks, when they are cooled from the high bonding temperature to room temperature, due to the thermal strain resulting from the difference in the respective thermal expansion coefficients of the terminals of the liquid-crystal device, the terminals being mounted on a glass plate, and the leads of the film carrier on which the device is mounted.
The more terminals the semiconductor element has, the more difficult it is to make them of the some height. In most cases, some terminals will be taller than others. Consequently, when the leads of a film carrier are pressed onto the bumps formed on the terminals of a semiconductor element, the pressure is concentratedly applied on the taller terminals. This may cause damage to the semiconductor element.
When tape automated bonding method is employed, the film carrier must be aligned with the substrate in order to transfer the bumps onto the leads of the film carrier. Thereafter, the film carrier with the transferred bumps must be aligned with an outer terminal. In other words, two alignment apparatuses are required, in order to perform tape automated bonding. Since an alignment apparatus is very expensive, the tape automated bonding method is, therefore, uneconomical.